| In recent years,the national energy strategy has been gradually deepened.Because of its mature power generation technology and considerable energy scale,offshore wind power has become one of the largest use of green energy.With the gradual expansion of the application scale of offshore wind power modular multilevel converters HVDC(MMC-HVDC),the proportion of synchronous power supply at the receiving end of the grid side gradually decreases.Under the same load disturbance,the grid frequency will shift significantly,which is not conducive to the safe and stable operation of the grid system.At the same time,the scale of offshore wind power utilization is huge enough to meet the demand of multi-terminal land receiving power grid.Multiterminal direct current based on modular multilevel converter will become the trend of offshore wind power grid connection in the future.Therefore,this paper constructs and proposes the mathematical model and corresponding control strategy of offshore wind power grid connection via MMC-HVDC,and studies the application of multi-terminal flexible DC transmission technology on this basis,and proposes the corresponding control strategy.(1)This article studies the topology of a doubly fed induction generator(DFIG)and establishes mathematical models and corresponding control models for each part.A mathematical model for the basic unit MMC(Modular Multilevel Converter,MMC)of MMC-HVDC was established,and the working mode of MMC was explained.A voltage control model was established for the MMC converter stations on the sending and receiving sides of offshore wind power connected to the grid through MMC-HVDC.The main factors affecting the inertia level and primary frequency regulation capability of the receiving power grid are studied,and the effects of different wind power ratios and frequency regulation parameters on the frequency stability of the receiving power grid are analyzed through simulation verification.(2)For the point-to-point MMC-HVDC grid-connected system of offshore wind power,a collaborative frequency control strategy is proposed to improve the grid inertia level and primary frequency modulation capability by using the wind power reserve and DC capacitance.Firstly,the influence factors of frequency response characteristics and frequency stability are studied;then the frequency-DC voltage equation is established,and the MMC-HVDC inertia support is realized by adjusting the DC voltage to control the DC capacitor to absorb or release energy;the grid frequency deviation and differential are introduced into the wind turbine power control link to realize the coordinated frequency control of wind turbine when the grid frequency changes.The results show that the proposed strategy can effectively improve the power grid inertia level and primary frequency modulation capability.(3)Based on the Multi-terminal MMC-HVDC grid-connected system of offshore wind power,a frequency-active power slope control strategy based on DC voltage-active power slope control and DC voltage as medium is proposed.Firstly,the frequency variation of the low inertia power grid is converted into the variation of DC voltage and introduced into the control link of the receiving large power grid MMC,and the multi-terminal frequency-active power slope control mode is constructed to realize the frequency support between AC power grids in the absence of communication.Then,a comprehensive frequency modulation control method is proposed that combines overspeed load shedding control,frequency modulation control,virtual inertia control on the low inertia grid side,and frequency active power slope control on the offshore wind turbine side.Construct a frequency response model of the receiving end low inertia power grid using a comprehensive frequency control strategy,and analyze the frequency response characteristics.The results show that the proposed strategy greatly improves the frequency response capability of the receiving power grid and the frequency support capability of the multi-receiving power grid. |
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